1. Field of the Invention
The present invention relates to the field of piezoelectric resonators, e.g. BAW (bulk acoustic wave) resonators, and particularly to a method of manufacturing an acoustic mirror for a piezoelectric resonator, as well as to a method of manufacturing a piezoelectric resonator. In particular, the present invention relates to a method of manufacturing an acoustic mirror, which is highly planar and has both excellent uniformity in the layer deposition and a planar surface of the entire mirror structure.
2. Description of the Related Art
Radio-frequency filters based on BAW resonators are of great interest for many RF applications. Substantially, there are two concepts for BAW resonators, so-called thin film BAW resonators (FBAR), on the one hand, as well as so-called solidly mounted resonators (SMR). Thin film BAW resonators include a membrane on which the layer sequence consisting of the lower electrode, the piezoelectric layer, and the upper electrode is arranged. The acoustic resonator develops by the reflection at the upper side and at the lower side of the membrane. In the alternative concept of solidly mounted resonators, an SMR includes a substrate, for example a silicon substrate, on which the layer sequence consisting of the lower electrode, the piezoelectric layer, and the upper electrode is arranged. So as to keep the acoustic waves in the active region in this design, a so-called acoustic mirror is required. It is located between the active layers, i.e. the two electrodes and the piezoelectric layer, and the substrate. The acoustic mirror consists of an alternating sequence of layers with high and low acoustic impedance, respectively, e.g. layers of tungsten (high acoustic impedance) and layers of oxide material (low acoustic impedance).
If the mirror contains layers of conducting materials, such as tungsten, it is recommended, for the avoidance of parasitic capacitances in the filter, to structure (pattern) and substantially limit the corresponding mirror layers to the area below the active resonator region. The disadvantage of this procedure is that the topology resulting hereby can no longer be completely planarized. Due to the unevenness, undesired modes are induced in the resonator and/or a reduction in the quality of the resonator is caused. This problem is very critical in so far as already small steps or remaining topologies of several percent of the layer thickness have significant influence on the operation behavior of such a resonator.
On the basis of FIGS. 1 and 2, two known methods of manufacturing acoustic mirrors for piezoelectric resonators or BAW resonators are explained in greater detail.
FIG. 1 shows a solidly mounted resonator with structured mirror. The resonator includes a substrate 100 with a lower surface 102 and an upper surface 104. A layer sequence 106 forming the acoustic mirror is arranged on the upper surface. Between the substrate and the mirror, one or more intermediate layers serving for stress reduction or adhesion improvement may be arranged, for example. The layer sequence includes alternately arranged layers 106a with high acoustic impedance and layers 106b with low acoustic impedance, wherein intermediate layers may be provided between the mirror layers. On the upper surface 104 of the substrate 100, a first layer 106b1 with low acoustic impedance is formed. On the layer 106b1, a material 106a1, 106a2 with high acoustic impedance is deposited and structured at the portions associated with the active regions of the resonator. Over this arrangement, a second layer 106b2 with low acoustic impedance is deposited, upon which in turn a material 106a3, 106a4 with high acoustic impedance is deposited and structured section-wise. Upon this layer sequence, again a layer with low acoustic impedance 106b3 is deposited. On the resulting mirror structure, a lower electrode 110, on which again the active or piezoelectric layer 112, for example of AlN, is arranged, is at least partially formed. On the piezoelectric layer 112, an insulation layer 114 covering the piezoelectric layer 112 except for the regions 116a and 116b is formed. Two upper electrodes 118a and 118b in contact with the piezoelectric layer in the portions 116a and 116b are formed on the piezoelectric layer. A tuning layer 120a and 120b, via the thickness of which a resonance frequency of the resonators can be adjusted, is at least partially arranged on the upper electrode 118a, 118b. By the portions of the upper electrode 118a and 118b in which it is in connection with the piezoelectric layer 112, and the underlying portions of the lower electrode 110, two BAW resonators 122a and 122b are defined. The mirror structure 106 shown in FIG. 1 includes λ/4 mirror layers 106a, 106b. 
In the example of a solidly mounted resonator shown in FIG. 1, as it is produced by Epcos AG, for example, the metallic layers 106a are structured without planarizing the resulting topology. The layers 106b with low acoustic impedance are deposited over the structured layers 106a, as described above. Thereby, the steps shown in FIG. 1, which continue in the deposition of the overlaying layers, develop. This procedure is disadvantageous regarding the resulting strong topology in the layers lying above the mirror 106, in particular, with reduced piezoelectric coupling of the active layer 112 as well as increased excitation of undesired vibrational modes arising.
FIG. 2 shows a further example known in the prior art for solidly mounted resonators with a structured mirror. In FIG. 2, again a substrate 100 is shown, on the upper surface 104 of which an oxide layer 124 is deposited, into which a pit or depression 126 is introduced. Further intermediate layers may be provided between the oxide layer 124 and the substrate 100. In the pit 126, the acoustic mirror is formed, which consists of a layer sequence comprising a first layer 106a1 with high acoustic impedance, a layer 106b with low acoustic impedance, and a layer 106a2 with high acoustic impedance. On the surface of the resulting structure, an insulation layer 108 is deposited, on which the lower electrode 110 is at least partially formed. The portion of the insulation layer 108 not covered by the lower electrode 110 is covered by a further insulation layer 128. On the insulation layer 128 and on the lower electrode 110, the piezoelectric layer 112 is formed, on the surface of which the upper electrode 118 is in turn partially formed. The portions of the piezoelectric layer 112 not covered by the upper electrode 118, as well as parts of the upper electrode 118 are covered by the passivation layer 114. The overlapping areas of lower electrode 110, piezoelectric layer 112, and upper electrode 118 define the BAW resonator 122.
In the example shown in FIG. 2, the pit 126, in which the mirror layers 106a, 106b are deposited after each other, as described above, is etched into the oxide layer 124 in the area of the resonator 122 to be produced. By one or more CMP (chemical mechanical polishing) processes, the layers outside the mirror pit 126 are removed, as this is described in the U.S. patent application US 2002/154425 A1, for example.
The method described on the basis of FIG. 2 is disadvantageous in that the layers are slightly thinner in the corners of the mirror pit 126, and a slight key topology in the resonator region 122, indicated with the reference numeral 130, develops, which again leads to increased excitation of undesired modes and to reduced resonator quality.